The present invention relates to a liquid crystal display device of a color light source type for providing a full-color display by causing a back-light to emit three colored lights in a time-divided manner.
Along with the recent developments of so-called office automation (OA), OA apparatuses such as word processors and personal computers have been widely used. Further, as such OA apparatuses have become prevalent in portable-type OA apparatuses that can be used in offices as well as outdoors, there have been demands for small-size and light-weight apparatuses. Liquid crystal display devices have been widely used as one of the means to meet such demands. In particular, liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable OA apparatuses that are driven by batteries.
By the way, the liquid crystal display devices are roughly classified into the reflection type and the transmission type. The reflection type liquid crystal display devices are constructed so that light rays incident on the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel and an image is visualized by the reflected light, while the transmission type liquid crystal display devices are constructed so that an image is visualized by transmitted light from a light source (back-light) provided on the rear face of the liquid crystal panel. Although the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount varying depending on environmental conditions, they have been widely used as monochrome (such as black-and-white) display devices for portable calculators, watches, etc. because of their low costs, but they are not suitable for the display devices of personal computers, etc. providing a multi-color or full-color display. For this reason, in general, transmission type liquid crystal display devices are used as display devices of personal computers, etc. providing a multi-color or full-color display.
Meanwhile, currently-used color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type based on the liquid crystal materials to be used. The STN type liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the response rate. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a battery power source. The TFT-TN type liquid crystal display devices have also problems such as a low response rate, particularly in displaying half tones, a narrow viewing angle, and a difficult color balance adjustment.
Additionally, conventional transmission type liquid crystal display devices are generally of the color-filter type which uses a back-light of white light and is designed to provide a multi-color or full-color display by selectively transmitting white light through color filters of the three primary colors. However, in such a color-filter type, since a display pixel is formed by a certain area including adjacent three color filters as one unit, the resolution is lowered to virtually one-third.
In view of the above-mentioned problems, there has been proposed a color liquid crystal display device (Japanese Patent Application Laid-Open No. 7-281150, etc.) which uses a ferroelectric liquid crystal element or an anti-ferroelectric liquid crystal element having a high response speed with respect to an applied electric field as its liquid crystal element and causes the same pixel to emit lights of the three primary colors in a time-divided manner so as not to cause a substantial lowering of the resolution.
With this color liquid crystal display device, it is possible to provide a color display by combining a liquid crystal panel using a ferroelectric liquid crystal element or an anti-ferroelectric liquid crystal element capable of responding at a high speed of several hundreds to several xcexc order with a back-light capable of emitting lights of red, green and blue colors in a time-divided manner and by synchronizing the switching of the liquid crystal element with the light emission of the back-light. In the case where a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal is used as the liquid crystal material, since the liquid crystal molecules are constantly maintained parallel to the substrate (glass substrate) regardless of the presence or absence of an applied voltage, a very wide viewing angle is obtained, and thus no problem arises in practical use. Moreover, in the case where a back-light constituted by red, green and blue light-emitting diodes (LEDs) is used, it is possible to adjust the color balance by controlling a current flowing through each LED.
FIG. 1 is a time chart showing one example of conventional display control in such a color liquid crystal display device, wherein FIG. 1(a) indicates the light-emitting timing of the LEDs of the respective colors of the back-light and FIG. 1(b) shows the scanning timing of the respective lines of the liquid crystal panel.
As shown in FIG. 1(a), the LEDs of the back-light are caused to emit red, green and blue lights sequentially in this order every 5.6 ms, for example, and the pixels of the liquid crystal panel are switched on a line by line basis in synchronism with the light emission so as to provide a display. Besides, in the case where 60 frames are displayed in one second, one frame period is 16.6 ms, and this one frame period is further divided into three sub-frames, each having a period of 5.6 ms, so that in the case illustrated in FIG. 1(a), for example, the red LED, the green LED and the blue LED emit light in the first sub-frame, the second sub-frame and the third sub-frame, respectively.
Meanwhile, as shown in FIG. 1(b), with respect to the liquid crystal panel, data scanning is performed twice within each of the sub-frames of the respective red, green and blue colors. However, timing adjustments are performed so that the start timing (timing for the first line) of the first scanning (data-writing scanning) coincides with the start timing of each sub-frame and the end timing (timing for the final line) of the second scanning (data-erasing scanning) coincides with the end timing of each sub-frame.
During the data-writing scanning, a voltage corresponding to pixel data is supplied to each pixel of the liquid crystal panel so as to adjust the transmittance. It is thus possible to provide a full-color display. During the data-erasing scanning, a voltage which is the same as the voltage for the data-writing scanning but has opposite polarity is supplied to each pixel of the liquid crystal panel so as to erase the display of the pixels of the liquid crystal panel, thereby preventing application of a direct-current component to the liquid crystal.
By the way, in such a liquid crystal panel, the intensity of the transmitted light varies according to write/erasure scanning of the liquid crystal panel. In other words, it has been confirmed by experiments that, even when the same voltage is applied, there is a difference in the light transmittance between the scanning start area and the scanning end area because of a problem associated with the liquid crystal panel""s own characteristics. FIGS. 2 and 3 are illustrations for explaining such a phenomenon: FIG. 2 shows the respective areas (areas 1-4) given by virtually dividing the display area of the liquid crystal panel into four areas together with the scanning direction; and FIG. 3 is a graph showing the applied voltage-light transmittance characteristics in the respective divided areas. Even when the same voltage is applied, the light transmittance becomes the largest in the area 1 located on the scanning start side, decreases gradually toward the downstream side of scanning, and becomes the smallest in the area 4 located on the scanning end side. Thus, there is a problem that the luminance is inconsistent within the entire display area.
The present invention was invented in view of such circumstances, and its object is to provide a liquid crystal display device having no inconsistency in the display luminance.
Another object of the present invention is to provide a liquid crystal display device capable of readily and selectively making the luminance in a certain display area higher than in other display areas without causing a considerable increase in power consumption.
A liquid crystal display device of the first aspect comprises: a liquid crystal panel having a plurality of liquid crystal pixels and a plurality of switching elements provided to correspond to the respective liquid crystal pixels; a back-light, disposed on the rear face of the liquid crystal panel, for emitting three colored lights in a time-divided manner; and a controller for providing a color display by driving the switching elements to be on/off according to data of three colors of the respective liquid crystal pixels, causing the back-light to emit the lights in a time-divided manner in synchronism with the on/off driving and scanning the respective liquid crystal pixels during the emission of the lights in a time-divided manner, wherein the light emitting area of the back-light is divided into a plurality of areas, and the intensity of light to be emitted from each of the divided light emitting areas differs from each other.
In the liquid crystal display device of the first aspect, the light emitting area of the back-light capable of emitting the three colored lights separately is divided into at least two areas, and the intensity of light to be emitted from each of the areas is arranged not to be the same. With scanning of the liquid crystal panel, the light transmittance in the display area of the liquid crystal panel varies. Therefore, the intensity of light to be emitted by the back-light is increased in an area corresponding to a display area with a low light transmittance, while the intensity of light is decreased in an area corresponding to a display area with a high light transmittance. Accordingly, even when there is a difference in the light transmittance, the display luminance becomes uniform over the entire area without causing inconsistency in the luminance.
A liquid crystal display device of the second aspect comprises a switching circuit for switching each of the divided light emitting areas of the back-light to emit light or put out light in synchronism with scanning of each of the liquid crystal pixels.
In the liquid crystal display device of the second aspect, the light-emitting timing in each of the light emitting areas of the back-light is controlled in synchronism with scanning of the liquid crystal panel. Therefore, by causing each of the light emitting areas to emit light only in a necessary period, it is possible to improve the utilization efficiency of the back-light.
In a liquid crystal display device of the third aspect, the back-light includes a light source divided into parts corresponding to the respective divided light emitting areas.
In the liquid crystal display device of the third aspect, the light source of the back-light is divided into parts corresponding to a plurality of light emitting areas. Therefore, by adjusting the intensity of light of each light source, it is possible to readily control the intensity of light to be emitted.
A liquid crystal display device of the fourth aspect comprises a control circuit for controlling the intensity of light to be emitted from each of the divided light emitting areas of the back-light in synchronism with scanning of each of the liquid crystal pixels.
In the liquid crystal display device of the fourth aspect, the intensity of light to be emitted from each of the light emitting areas of the back-light is controlled in synchronism with scanning of the liquid crystal panel. It is therefore possible to compensate for differences in the light transmittance resulting from scanning of the liquid crystal panel.
A liquid crystal display device of the fifth aspect comprises a control circuit for controlling the intensity of light to be emitted from each of the divided light emitting areas of the back-light according to a light transmittance of each of display areas of the liquid crystal panel corresponding to each of the divided light emitting areas.
In the liquid crystal display device of the fifth aspect, the intensity of light to be emitted from each of the divided light emitting areas of the back-light is controlled according to the light transmittance of the liquid crystal panel. It is therefore possible to accurately compensate for differences in the light transmittance resulting from scanning of the liquid crystal panel. Moreover, it is possible to readily achieve a higher luminance in a certain display area than in other display areas without causing a considerable increase in power consumption.
In a liquid crystal display device of the sixth aspect, the light-emitting time of each of the three colored lights is not more than {fraction (1/180)} second.
In the liquid crystal display device of the sixth aspect, the image display of one frame is completed within a time of not more than {fraction (1/60)} second, thereby enabling display of 60 or more frames per second.
In a liquid crystal display device of the seventh aspect, the back-light comprises LEDs for emitting three colored lights, respectively, a diffusing plate for diffusing light emitted by the LEDs, and a light guiding plate for guiding light emitted by the LEDs to one face of the liquid crystal panel.
In the liquid crystal display device of the seventh aspect, since the back-light is composed of the LEDs of the respective three colors (red, green and blue), the diffusing plate for diffusing light emitted by the respective LEDs and the light guiding plate for guiding light emitted by the respective LEDs to one face of the liquid crystal panel, the transmitted light from the back-light is uniform.
In a liquid crystal display device of the eighth aspect, a liquid crystal material of the liquid crystal panel is a ferroelectric liquid crystal material or an antiferroelectric liquid crystal material.
In the liquid crystal display device of the eighth aspect, since the liquid crystal material is a ferroelectric liquid crystal material or an antiferroelectric liquid crystal material, it is possible to perform high-speed on/off control and sufficiently correspond to control of the light emission of the back-light.